1. Field of the Invention
This invention relates to a control system for an automatic transmission of the type having a planetary gearset and a hydraulically actuated clutch that selectively holds components of the gearset to produce the various drive ratios of the transmission. More particularly, the present invention relates to a control system having two feedback loops, one loop for producing the torque carried by the clutch in accordance with the difference between the commanded and actual power source speeds and a second loop for producing a power source torque in accordance with the difference between commanded and actual drive wheel torques.
2. Description of the Prior Art
Hydraulically actuated, mechanically driven automatic transmissions generally include a turbine driven hydrodynamically from the impeller of a torque converter, the turbine transmitting power to the gearset to produce the forward drive gear ratios. When any forward or reverse gear ratio is selected, though the vehicle is coasting or standing at idle, the engine transmits torque through the torque converter, various clutches and gears to the drive wheels. Automatic transmissions can have the gear ratio changes produced as a result of repetitively executing an algorithm that determines when the transmission is to be shifted and the gear ratio to which the gear shift is to be made.
To improve the efficiency of the powertrain, it is desirable to eliminate the torque converter used in an automatic transmission and to schedule the shifts using an onboard computer to optimize the efficiency of the power source whether it be an internal combustion engine or an electric motor. Because the shifts are scheduled by the computer and not initiated or expected by the driver, and because there is no torque converter to absorb sudden torque changes, it is essential that the shift sequence and the torque during the shift be carefully controlled to minimize the wheel torque changes during the shift. The control technique, according to this invention, has been developed to control the power source torque, and the torque transmitted by the clutch of an automatic transmission to achieve smooth shifts in an acceptably brief period so that the service life of the clutch is consistent with the service life of the transmission.